In general, a relatively high transmission power level is required for proper communications between a remote mobile station and a base station of a mobile communications system employing, e.g., W-CDMA (Wideband Code Division Multiple Access) scheme. In such a case, communication signals sometimes need be highly amplified at a transmission end, e.g., the base station.
An amplifier, which is an analog device, generally exhibits nonlinear input/output characteristics. Specifically, the output power of the amplifier becomes almost constant even while an input power of the amplifier increases beyond an amplification limit referred to as a saturation point. Such nonlinear output leads to a nonlinear distortion in the output signal.
Typically, spurious emissions out of a band signal are suppressed to a low power level by a band-pass filter before the transmission signal is amplified. However, after the transmission signal is amplified at the amplifier, the amplified signal exhibits a nonlinear distortion, entailing a leak of a part of the amplified signal into adjacent channels. Since the transmission power level at the base station can be high as described above, the leakage power level to the adjacent channels need be suppressed below a certain strictly defined level. To that end, techniques for reducing ACP (Adjacent Channel leakage Power) have been used in the conventional amplifiers.
In order to reduce the ACP, there are employed various methods such as a back-off, a feed-forward and a predistortion method for the amplifier.
Referring to FIGS. 7A and 7B, there are shown characteristics of the amplifier and operation characteristics of the methods described above. FIGS. 7A and 7B show control characteristics of a back-off method and of a feed-forward and a predistortion method, respectively.
The back-off method prevents a nonlinear distortion from being generated by limiting an operation range of the amplifier within a linear region thereof by way of lowering an operating point. To be specific, the operating point of the back-off method is set at a point below a maximum output power level maintaining the linearity, for example, at a point lower than the maximum level by as much as the magnitude of the output power corresponding to a peak factor of an input signal of the amplifier, as shown in FIG. 7A.
The peak factor is the ratio of the maximum power level of the input signal of the amplifier to the average power level thereof as shown in FIG. 8. That is, the peak factor becomes smaller as the difference between the maximum power level and the average power level is reduced.
On the other hand, in the feed-forward method, distortion compensation is performed by subtracting a distortion component amplified in an error amplifier from an output of a main amplifier containing therein a desired signal and the distortion component.
In the predistortion method, a predistortion component is introduced to an input signal of an amplifier in order to compensate the distortion to be produced in the output of the amplifier. The predistortion component is determined based on the characteristics opposite to the nonlinear characteristics to be generated by, e.g., an AM/AM and an AM/PM conversion process in the amplifier.
As described above, the feed-forward and the predistortion method can be used in effectively removing the distortion generated by an amplifier. Therefore, as shown in FIG. 7B, an operating point in either method can be set at a point below a saturation output power level of the amplifier, the operating point being set lower than the saturation level by the magnitude of the output power corresponding to a peak factor, as shown in FIG. 7B. Therefore, the operating point of the feed-forward and the predistortion method can be set higher than that of the back-off method.
Thus, the feed-forward and the predistortion method are widely used since it is possible to improve the power efficiency of the system by employing them. This higher operating point leads to a greater output power to thereby improve the power efficiency. The power efficiency of an amplifier also greatly depends on the magnitude of the peak factor of the input signal of the amplifier.
A larger peak factor requires a greater size of a transistor to be used in the power amplifier circuit. Moreover, the operating point of the amplifier should be lowered greatly to avoid the saturation of the output power level, so that the ratio of the output power to a DC input power of the amplifier is reduced.
For these reasons, in order to improve the power efficiency of the amplifier, the peak factor need be controlled to be as small as possible. To this end, a peak limiter is introduced, e.g., at a front end of the amplifier to provide a maximum power (“peak”) limited signal as an input signal of the amplifier.
A conventional peak limiter will now be described hereinafter with reference to FIG. 10. Referring to FIG. 10, there is provided a block diagram showing an exemplary structure of a conventional peak limiter.
The conventional peak limiter 1′ includes an average power detection unit 11′, an instantaneous power detection unit 12′, a peak detection unit 13′ and a limiter unit 14′ as shown in FIG. 10.
The average power detection unit 11′ detects the average power of an input signal (I, Q) of the peak limiter 1′ to thereby output average power information.
The instantaneous power detection unit 12′ detects instantaneous power levels of the input signal (I, Q) to output instantaneous power information.
The peak detection unit 13′ detects whether a peak to be power-limited exists in the input signal (I, Q). Specifically, the peak detection unit 13′ calculates instantaneous-to-average power ratios of the input signal, an instantaneous-to-average power ratio representing the ratio of an instantaneous power level from the instantaneous power detection unit 12′ to an average power level from the average power detection unit 11′. Then, the peak detection unit 13′ detects whether there exists an instantaneous-to-average power ratio exceeding a predetermined peak threshold value. The portions of the input signal corresponding to the instantaneous-to-average power ratio exceeding the peak threshold value are determined as a peak to be power-limited and the detection result is outputted as peak detection information to the limiter unit 14′. The predetermined peak threshold value corresponds to the peak factor considered in determining the operating point in FIG. 7B.
The limiter unit 14′ limits the power of the input signal (I, Q) in response to the peak detection information. To be specific, when a peak having the instantaneous-to-average power ratio exceeding the peak threshold value is detected by the peak detection unit 13′, the limiter unit 14′ in response thereto limits the power of the peak not to exceed a predetermined level. Therefore, the input signal (I, Q) is controlled to have a maximum power lowered down to the predetermined level (limit power). Thus peak-limited input signal (I, Q) is provided from the limiter unit 14′ as an output signal (I, Q) thereof.
In general, the input signal (I, Q) fed to the peak limiter 1′ is a baseband signal which is not band-limited yet. The peak limitation by the peak limiter 1′ is performed for such input signal (I, Q) and then filtering by a low pass filter (not shown) is carried out therefor, so that distortion generation can be prevented. Moreover, since the peak of the input signal is limited by the peak limiter 1′, the peak factor of the input signal can be made to be small, which can lead to a high operating point of the amplifier to improve the power efficiency thereof.
The peak factor of a signal normally becomes greater after being subjected to band limitation. This is because the input square wave rounded after band limitation, rendering certain portions of the peaks of the signal to increase. For this reason, the peak threshold value set in the peak detection unit 13′ need be determined to be conservatively low by considering the peak factor increment after the band limitation.
Referring to FIG. 11, there is illustrated a schematic block diagram of a conventional multi-carrier amplification apparatus employing the prior art peak limiter 1′ shown in FIG. 10, wherein it is assumed that there exist two carriers in a multi-carrier signal.
In the conventional multi-carrier amplification apparatus, provided for the two carriers are their respective peak limiters 1′-1 and 1′-2, each for peak limiting its corresponding input signal (I, Q) −1 or −2, filters 2-1 and 2-2 for performing the band limitation of respective peak-limited signals A1 and A2 and up-converters 3-1 and 3-2 for up-converting the respective band-limited signals B1 and B2 to RF band (high frequency modulation) to provide carrier signals C1 and C2. The conventional multi-carrier amplification apparatus further includes a coupler 4 for coupling the carrier signals C1 and C2 to output a multi-carrier signal D and an amplifier 5 for amplifying the multi-carrier signal D to generate an output signal (C1, C2).
To be specific, the peak limiters 1′-1 and 1′-2 in the conventional multi-carrier amplification apparatus calculate the instantaneous-to-average power ratios of their corresponding input signals to output the peak-limited signals A1 and A2 in a manner described with respect to the peak limiter 1′ shown in FIG. 10. The filters 2-1 and 2-2 perform filtering on the peak-limited input signals A1 and A2 to output the band-limited signals B1 and B2. The up-converters 3-1 and 3-2 up-convert the band-limited signals B1 and B2 to the radio frequency (RF) band to thereby output the up-converted RF modulated signals (carrier signals) C1 and C2. Then, the coupler 4 combines the carrier signals C1 and C2 to output a multi-carrier signal D. The amplifier 5 amplifies the multi-carrier signal D to generate the amplified multi-carrier signal as the output signal (C1, C2) thereof.
As described, since peak factors of the input signals (I, Q) are reduced by the peak limiters 1′-1 and 1′-2, a peak factor of the multi-carrier signal D can be also suppressed. As a result, the operating point of the amplifier 5 can be raised to enhance the power efficiency thereof.
Japanese Patent Laid Open No. 2000-244452 entitled “CDMA wireless base station” discloses a technique capable of reducing the generation of distortion during amplifying a multi-carrier signal.
In the above CDMA wireless base station of the prior art, a limit level of a baseband input signal is set to be high in a case where a modulation signal having a large number of carriers and great transmission power is inputted to a common amplifier. In other cases, however, the limit level of the baseband input signal is set to be low. Through such control, the distortion generation can be lowered in case of having a large number of carriers and great transmission power and an error rate at a reception end can be reduced in case of a small number of the carriers. Moreover, since an amplifier having a low power consumption can be employed by adaptively changing the limit level, the overall power consumption of the entire amplification apparatus can also be reduced.
However, in the prior art peak limiter and multi-carrier amplification apparatus, the peak limitation is carried out with respect to the individual input signal based on a peak factor and a peak threshold value of each input signal; and the peak-limited signals are combined after band limitation. As a result, the peak factor of the multi-carrier signal outputted from the coupler 4 becomes generally greater than that of each of the peak-limited signals A1 and A2, so that it is difficult to properly adjust the predetermined peak threshold value in order to obtain a desired peak factor for the input signal fed to the amplifier 5. Therefore, the peak threshold value is predetermined conservatively low for the sake of safety, so that effective peak limitation can not be performed. For example, a peak factor of a multi-carrier signal having 2 to 4 carriers in W-CDMA can be greater by 2-6 dB than that of a multi-carrier signal having one carrier.
Moreover, even in a case where an actual power of the multi-carrier signal coupled at the coupler 4 is small, the peak limitation is performed on individual input signal if instantaneous-to-average power ratios thereof are greater than the predetermined peak threshold value, resulting in unnecessary peak limitation, which in turn deteriorates the modulation accuracy.